The collapse of the entire food chain? Insect biomass is down 80% in the last 40 years. Ocean phytoplankton is down 40% since 1950. Both are attributed to climate change. These things are at the bottom of the land and sea food chains respectively. If the food chain collapses completely, we will all starve.

This is not a gradual change of the type we need to learn to adapt to. This is a sudden, full-blown, in-progress apocalypse of a scale only seen once before in Earth's history.